---
title: Zod Core
description: "Zod Core package - minimal core functionality for custom implementations"
---

import { Callout } from "fumadocs-ui/components/callout"
import { Accordion, Accordions } from 'fumadocs-ui/components/accordion';

This sub-package exports the core classes and utilities that are consumed by Zod and Zod Mini. It is not intended to be used directly; instead it's designed to be extended by other packages. It implements:

```ts
import * as z from "zod/v4/core";

// the base class for all Zod schemas
z.$ZodType;

// subclasses of $ZodType that implement common parsers
z.$ZodString
z.$ZodObject
z.$ZodArray
// ...

// the base class for all Zod checks
z.$ZodCheck;

// subclasses of $ZodCheck that implement common checks
z.$ZodCheckMinLength
z.$ZodCheckMaxLength

// the base class for all Zod errors
z.$ZodError;

// issue formats (types only)
{} as z.$ZodIssue;

// utils
z.util.isValidJWT(...);
```

## Schemas

The base class for all Zod schemas is `$ZodType`. It accepts two generic parameters: `Output` and `Input`.

```ts
export class $ZodType<Output = unknown, Input = unknown> {
  _zod: { /* internals */}
}
```

`zod/v4/core` exports a number of subclasses that implement some common parsers. A union of all first-party subclasses is exported as `z.$ZodTypes`.

```ts
export type $ZodTypes =
  | $ZodString
  | $ZodNumber
  | $ZodBigInt
  | $ZodBoolean
  | $ZodDate
  | $ZodSymbol
  | $ZodUndefined
  | $ZodNullable
  | $ZodNull
  | $ZodAny
  | $ZodUnknown
  | $ZodNever
  | $ZodVoid
  | $ZodArray
  | $ZodObject
  | $ZodUnion // $ZodDiscriminatedUnion extends this
  | $ZodIntersection
  | $ZodTuple
  | $ZodRecord
  | $ZodMap
  | $ZodSet
  | $ZodLiteral
  | $ZodEnum
  | $ZodPromise
  | $ZodLazy
  | $ZodOptional
  | $ZodDefault
  | $ZodTemplateLiteral
  | $ZodCustom
  | $ZodTransform
  | $ZodNonOptional
  | $ZodReadonly
  | $ZodNaN
  | $ZodPipe // $ZodCodec extends this
  | $ZodSuccess
  | $ZodCatch
  | $ZodFile;
```

<Accordions>
<Accordion title="Inheritance diagram">
  Here is a complete inheritance diagram for the core schema classes:
  ```txt
  - $ZodType
      - $ZodString
          - $ZodStringFormat
              - $ZodGUID
              - $ZodUUID
              - $ZodEmail
              - $ZodURL
              - $ZodEmoji
              - $ZodNanoID
              - $ZodCUID
              - $ZodCUID2
              - $ZodULID
              - $ZodXID
              - $ZodKSUID
              - $ZodISODateTime
              - $ZodISODate
              - $ZodISOTime
              - $ZodISODuration
              - $ZodIPv4
              - $ZodIPv6
              - $ZodCIDRv4
              - $ZodCIDRv6
              - $ZodBase64
              - $ZodBase64URL
              - $ZodE164
              - $ZodJWT
      - $ZodNumber
          - $ZodNumberFormat
      - $ZodBigInt
          - $ZodBigIntFormat
      - $ZodBoolean
      - $ZodSymbol
      - $ZodUndefined
      - $ZodNull
      - $ZodAny
      - $ZodUnknown
      - $ZodNever
      - $ZodVoid
      - $ZodDate
      - $ZodArray
      - $ZodObject
      - $ZodUnion
          - $ZodDiscriminatedUnion
      - $ZodIntersection
      - $ZodTuple
      - $ZodRecord
      - $ZodMap
      - $ZodSet
      - $ZodEnum
      - $ZodLiteral
      - $ZodFile
      - $ZodTransform
      - $ZodOptional
      - $ZodNullable
      - $ZodDefault
      - $ZodPrefault
      - $ZodNonOptional
      - $ZodSuccess
      - $ZodCatch
      - $ZodNaN
      - $ZodPipe
          - $ZodCodec
      - $ZodReadonly
      - $ZodTemplateLiteral
      - $ZodCustom

      ```
</Accordion>
</Accordions>

## Internals

All `zod/v4/core` subclasses only contain a single property: `_zod`. This property is an object containing the schemas *internals*. The goal is to make `zod/v4/core` as extensible and unopinionated as possible. Other libraries can "build their own Zod" on top of these classes without `zod/v4/core` cluttering up the interface. Refer to the implementations of `zod` and `zod/mini` for examples of how to extend these classes. 

The `_zod` internals property contains some notable properties:

- `.def` — The schema's *definition*: this is the object you pass into the class's constructor to create an instance. It completely describes the schema, and it's JSON-serializable.
  - `.def.type` — A string representing the schema's type, e.g. `"string"`, `"object"`, `"array"`, etc.
  - `.def.checks` — An array of *checks* that are executed by the schema after parsing.
- `.input` — A virtual property that "stores" the schema's *inferred input type*.
- `.output` — A virtual property that "stores" the schema's *inferred output type*.
- `.run()` — The schema's internal parser implementation.

If you are implementing a tool (say, a code generator) that must traverse Zod schemas, you can cast any schema to `$ZodTypes` and use the `def` property to discriminate between these classes.

```ts
export function walk(_schema: z.$ZodType) {
  const schema = _schema as z.$ZodTypes;
  const def = schema._zod.def;
  switch (def.type) {
    case "string": {
      // ...
      break;
    }
    case "object": {
      // ...
      break;
    }
  }
}
```

There are a number of subclasses of `$ZodString` that implement various *string formats*. These are exported as `z.$ZodStringFormatTypes`.

```ts
export type $ZodStringFormatTypes =
  | $ZodGUID
  | $ZodUUID
  | $ZodEmail
  | $ZodURL
  | $ZodEmoji
  | $ZodNanoID
  | $ZodCUID
  | $ZodCUID2
  | $ZodULID
  | $ZodXID
  | $ZodKSUID
  | $ZodISODateTime
  | $ZodISODate
  | $ZodISOTime
  | $ZodISODuration
  | $ZodIPv4
  | $ZodIPv6
  | $ZodCIDRv4
  | $ZodCIDRv6
  | $ZodBase64
  | $ZodBase64URL
  | $ZodE164
  | $ZodJWT
```


## Parsing 

As the Zod Core schema classes have no methods, there are top-level functions for parsing data.

```ts
import * as z from "zod/v4/core";

const schema = new z.$ZodString({ type: "string" });
z.parse(schema, "hello");
z.safeParse(schema, "hello");
await z.parseAsync(schema, "hello");
await z.safeParseAsync(schema, "hello");
```


## Checks

Every Zod schema contains an array of *checks*. These perform post-parsing refinements (and occasionally mutations) that *do not affect* the inferred type.

```ts
const schema = z.string().check(z.email()).check(z.min(5));
// => $ZodString

schema._zod.def.checks;
// => [$ZodCheckEmail, $ZodCheckMinLength]
```

The base class for all Zod checks is `$ZodCheck`. It accepts a single generic parameter `T`.

```ts
export class $ZodCheck<in T = unknown> {
  _zod: { /* internals */}
}
```

The `_zod` internals property contains some notable properties:

- `.def` — The check's *definition*: this is the object you pass into the class's constructor to create the check. It completely describes the check, and it's JSON-serializable.
  - `.def.check` — A string representing the check's type, e.g. `"min_length"`, `"less_than"`, `"string_format"`, etc.
- `.check()` — Contains the check's validation logic.

`zod/v4/core` exports a number of subclasses that perform some common refinements. All first-party subclasses are exported as a union called `z.$ZodChecks`.

```ts
export type $ZodChecks =
  | $ZodCheckLessThan
  | $ZodCheckGreaterThan
  | $ZodCheckMultipleOf
  | $ZodCheckNumberFormat
  | $ZodCheckBigIntFormat
  | $ZodCheckMaxSize
  | $ZodCheckMinSize
  | $ZodCheckSizeEquals
  | $ZodCheckMaxLength
  | $ZodCheckMinLength
  | $ZodCheckLengthEquals
  | $ZodCheckProperty
  | $ZodCheckMimeType
  | $ZodCheckOverwrite
  | $ZodCheckStringFormat
```


You can use the `._zod.def.check` property to discriminate between these classes.

```ts
const check = {} as z.$ZodChecks;
const def = check._zod.def;

switch (def.check) {
  case "less_than":
  case "greater_than":
    // ...
    break;
}
```


As with schema types, there are a number of subclasses of `$ZodCheckStringFormat` that implement various *string formats*. 

```ts
export type $ZodStringFormatChecks =
  | $ZodCheckRegex
  | $ZodCheckLowerCase
  | $ZodCheckUpperCase
  | $ZodCheckIncludes
  | $ZodCheckStartsWith
  | $ZodCheckEndsWith
  | $ZodGUID
  | $ZodUUID
  | $ZodEmail
  | $ZodURL
  | $ZodEmoji
  | $ZodNanoID
  | $ZodCUID
  | $ZodCUID2
  | $ZodULID
  | $ZodXID
  | $ZodKSUID
  | $ZodISODateTime
  | $ZodISODate
  | $ZodISOTime
  | $ZodISODuration
  | $ZodIPv4
  | $ZodIPv6
  | $ZodCIDRv4
  | $ZodCIDRv6
  | $ZodBase64
  | $ZodBase64URL
  | $ZodE164
  | $ZodJWT;
```


Use a nested `switch` to discriminate between the different string format checks.

```ts
const check = {} as z.$ZodChecks;
const def = check._zod.def;

switch (def.check) {
  case "less_than":
  case "greater_than":
  // ...
  case "string_format":
    {
      const formatCheck = check as z.$ZodStringFormatChecks;
      const formatCheckDef = formatCheck._zod.def;

      switch (formatCheckDef.format) {
        case "email":
        case "url":
          // do stuff
      }
    }
    break;
}
```

You'll notice some of these string format *checks* overlap with the string format *types* above. That's because these classes implement both the `$ZodCheck` and `$ZodType` interfaces. That is, they can be used as either a check or a type. In these cases, both `._zod.parse` (the schema parser) and `._zod.check` (the check validation) are executed during parsing. In effect, the instance is prepended to its own `checks` array (though it won't actually exist in `._zod.def.checks`).

```ts
// as a type
z.email().parse("user@example.com");

// as a check
z.string().check(z.email()).parse("user@example.com")
```

## Errors

The base class for all errors in Zod is `$ZodError`.

> For performance reasons, `$ZodError` *does not* extend the built-in `Error` class! So using `instanceof Error` will return `false`.

- The `zod` package implements a subclass of `$ZodError` called `ZodError` with some additional convenience methods.
- The `zod/mini` sub-package directly uses `$ZodError`

```ts
export class $ZodError<T = unknown> implements Error {
 public issues: $ZodIssue[];
}
```

## Issues

The `issues` property corresponds to an array of `$ZodIssue` objects. All issues extend the `z.$ZodIssueBase` interface.

```ts
export interface $ZodIssueBase {
  readonly code?: string;
  readonly input?: unknown;
  readonly path: PropertyKey[];
  readonly message: string;
}
```

Zod defines the following issue subtypes:

```ts
export type $ZodIssue =
  | $ZodIssueInvalidType
  | $ZodIssueTooBig
  | $ZodIssueTooSmall
  | $ZodIssueInvalidStringFormat
  | $ZodIssueNotMultipleOf
  | $ZodIssueUnrecognizedKeys
  | $ZodIssueInvalidUnion
  | $ZodIssueInvalidKey
  | $ZodIssueInvalidElement
  | $ZodIssueInvalidValue
  | $ZodIssueCustom;
  ```

For details on each type, refer to [the implementation](https://github.com/colinhacks/zod/blob/main/packages/zod/src/v4/core/errors.ts).


{/* ## Best practices

If you're reading this page, you're likely trying to build some kind of tool or library on top of Zod. This section breaks down some best practices for doing so.

1. If you're just accept user-defined schemas, use Standard Schema instead

Zod implements the [Standard Schema](https://standardschema.dev/) specification, a standard interface for schema libraries to expose their validation logic and inferred types to third-party tools. If your goal is to accept user-defined schemas, extracting their inferred types, and using them to parse data, then Standard Schema is all you need. Refer to the Standard Schema website/docs for more information.

2. Set up `peerDependencies` properly!

If your tool accepts Zod schemas from a consumer/user, you should add `"zod/v4/core"` to `peerDependencies`. This lets your users "bring their own Zod". Be as flexible as possible with the version range. For example, if your tool is compatible with `zod/v4/core`, you can use the following. This allows your users to bring any version of `zod/v4/core`, avoiding accidental duplicate installs.


```json
{
  "peerDependencies": {
    "zod/v4/core": "*"
  }
}
```

Since package managers generally won't install your own `peerDependencies`, you'll need to add `zod/v4/core` to your `devDependencies` as well. As new versions of `zod/v4/core` are released, you can update your `devDependencies` to match the latest version. This is important for testing and development purposes.

```json
{
  "peerDependencies": {
    "zod": "*"
  },
  "devDependencies": {
    "zod": "^3.25.0"
  }
}
``` */}

